The outer ear includes the pinna, the visible part of the ear, as well as the ear canal which terminates at the eardrum, also called the tympanic membrane. The pinna serves to focus sound waves through the ear canal toward the eardrum. Because of the asymmetrical character of the outer ear of most mammals, sound is filtered differently on its way into the ear depending on what vertical location it is coming from. This gives these animals the ability to localize sound vertically. The eardrum is an airtight membrane, and when sound waves arrive there, they cause it to vibrate following the waveform of the sound.

The middle ear consists of a small air-filled chamber that is located medial to the eardrum. Within this chamber are the three smallest bones in the body, known collectively as the ossicles which include the malleus, incus and stapes (sometimes referred to colloquially as the hammer, anvil and stirrup respectively). They aid in the transmission of the vibrations from the eardrum to the inner ear. The purpose of the middle ear ossicles is to overcome the impedance mismatch between air and water, by providing impedance matching.

Also located in the middle ear are the stapedius and tensor tympani muscles which protect the hearing mechanism through a stiffening reflex. The stapes transmits sound waves to the inner ear through the oval window, a flexible membrane separating the air-filled middle ear from the fluid-filled inner ear. The round window, another flexible membrane, allows for the smooth displacement of the inner ear fluid caused by the entering sound waves.

The inner ear consists of the cochlea, which is a spiral-shaped, fluid-filled tube. It is divided lengthwise by the organ of Corti, which is the main organ of mechanical to neural transduction. Inside the organ of Corti is the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid – endolymph. The basilar membrane is tonotopic, so that each frequency has a characteristic place of resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, and low at the apex. Basilar membrane motion causes depolarization of the hair cells, specialized auditory receptors located within the organ of Corti.[5] While the hair cells do not produce action potentials themselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials. In this way, the patterns of oscillations on the basilar membrane are converted to spatiotemporal patterns of firings which transmit information about the sound to the brainstem.[6]

Disturbances (such as stroke or trauma) at any of these levels can cause hearing problems, especially if the disturbance is bilateral. In some instances it can also lead to auditory hallucinations or more complex difficulties in perceiving sound.

The hearing structures of many species have defense mechanisms against injury. For example, the muscles of the middle ear (e.g. the tensor tympani muscle) in many mammals contract reflexively in reaction to loud sounds which may otherwise injure the hearing ability of the organism.

Mild hearing loss - People with mild hearing loss have difficulties keeping up with conversations, especially in noisy surroundings. The most quiet sounds that people with mild hearing loss can hear with their better ear are between 25 and 40 dB HL.

Moderate hearing loss - People with moderate hearing loss have difficulty keeping up with conversations when they are not using a hearing aid. On average, the most quiet sounds heard by people with moderate hearing loss with their better ear are between 40 and 70 dB HL.

Severe hearing loss - People with severe hearing loss depend on powerful hearing aid. However, they often rely on lip-reading even when they are using hearing aids. The most quiet sounds heard by people with severe hearing loss with their better ear are between 70 and 95 dB HL.

Profound hearing loss - People with profound hearing loss are very hard of hearing and they mostly rely on lip-reading and sign language. The most quiet sounds heard by people with profound hearing loss with their better ear are from 95 dB HL or more.

Hearing protection is the use of devices designed to prevent Noise-Induced Hearing Loss (NIHL), a type of post-lingual hearing impairment. The various means used to prevent hearing loss generally focus on reducing the levels of noise to which people are exposed. One way this is done is through environmental modifications such as acoustic quieting, which may be achieved with as basic a measure as lining a room with curtains, or as complex a measure as employing an anechoic chamber, which absorbs nearly all sound. Another means is the use of devices such as earplugs, which are inserted into the ear canal to block noise, or earmuffs, objects designed to cover a person's ears entirely.

Hearing aids are electronic devices that enable a person with hearing loss to receive sounds at certain amplitudes. This technological development has led to the benefit of improving the sense of hearing of a person, but the usage of these devices is significantly low. Psychologically, the first time that a person realizes that he/she needs help from a professional such as an audiologist is when they feel that their hearing is severely poor. Initially, people don't like to believe that they are becoming deaf; hence it negatively affects their approach towards the use of hearing aids. Familiarity with the devices and consultation with professionals do help people feel good about using the hearing aids.[9]

Hearing threshold and the ability to localize sound sources are reduced underwater in humans but not in aquatic animals, including whales, seals, and fishes which have ears adapted to process water borne sound.[10][11][12] Some research suggests underwater hearing in humans may occur through bone conduction but with poor localization. This is related to differences of the speed of sound in water vs air and the blocking of normal air conducted sound paths.[13]

A cat can hear high-frequency sounds up to two octaves higher than a human.

Not all sounds are normally audible to all animals. Each species has a range of normal hearing for both amplitude and frequency. Many animals use sound to communicate with each other, and hearing in these species is particularly important for survival and reproduction. In species that use sound as a primary means of communication, hearing is typically most acute for the range of pitches produced in calls and speech.

Frequencies capable of being heard by humans are called audio or sonic. The range is typically considered to be between 20 Hz and 20,000 Hz.[14] Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic. Some bats use ultrasound for echolocation while in flight. Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. Snakes sense infrasound through their jaws, and baleen whales, giraffes, dolphins and elephants use it for communication. Some fish have the ability to hear more sensitively due to a well-developed, bony connection between the ear and their swim bladder. The "aid to the deaf" of fishes appears in some species such as carp and herring.[15]

Something widely spread among insects is body hair, that can be made to swing by sonar waves. Due to the resonance phenomenon certain hairs swing stronger when exposed to a specific sonar-frequency. This specificity depends on the stiffness and the length of the hairs. That is why certain caterpillar species have evolved hair that would resonate with the sound of buzzing wasps, thus warning them of the presence of natural enemies. Moreover, mosquitoes have hair on their antennae that resonate with the flying sound of homogeneous females, enabling the males the ability to detect potential sexual partners.

Some insects possess a tympanal organ. These are "eardrums", that cover air filled chambers on the legs. Similar to the hearing process with vertebrates, the eardrums react to sonar waves. Receptors that are placed on the inside translate the oscillation into electric signals and send them to the brain. Several groups of flying insects that are preyed upon by echolocatingbats can perceive the ultrasound emissions this way and reflexively practice ultrasound avoidance.

The basilar membrane of the inner ear spreads out different frequencies: high frequencies produce a large vibration at the end near the middle ear (the "base"), and low frequencies a large vibration at the distant end (the "apex"). Thus the ear performs a sort of frequency analysis, roughly similar to a Fourier transform.[16][17] However, the nerve pulses delivered to the brain contain both rate-versus-place and fine temporal structure information, so the similarity is not strong.

1.
Sound
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In physics, sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a transmission medium such as air or water. In physiology and psychology, sound is the reception of such waves, humans can hear sound waves with frequencies between about 20 Hz and 20 kHz. Sound above 20 kHz is ultrasound and below 20 Hz is infrasound, other animals have different hearing ranges. Acoustics is the science that deals with the study of mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound. A scientist who works in the field of acoustics is an acoustician, an audio engineer, on the other hand, is concerned with the recording, manipulation, mixing, and reproduction of sound. Auditory sensation evoked by the oscillation described in, sound can propagate through a medium such as air, water and solids as longitudinal waves and also as a transverse wave in solids. The sound waves are generated by a source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium, as the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure, velocity, at an instant in time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel with the sound wave and this is intuitively obvious for a solid, and the same is true for liquids and gases. During propagation, waves can be reflected, refracted, or attenuated by the medium, the behavior of sound propagation is generally affected by three things, A complex relationship between the density and pressure of the medium. This relationship, affected by temperature, determines the speed of sound within the medium, if the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the wave will be decreased by the speed of the wind. Medium viscosity determines the rate at which sound is attenuated, for many media, such as air or water, attenuation due to viscosity is negligible. When sound is moving through a medium that does not have constant physical properties, the mechanical vibrations that can be interpreted as sound can travel through all forms of matter, gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium, sound cannot travel through a vacuum. Sound is transmitted through gases, plasma, and liquids as longitudinal waves and it requires a medium to propagate

2.
Vibration
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Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin vibrationem, the oscillations may be periodic, such as the motion of a pendulum—or random, such as the movement of a tire on a gravel road. Vibration can be desirable, for example, the motion of a fork, the reed in a woodwind instrument or harmonica. In many cases, however, vibration is undesirable, wasting energy, for example, the vibrational motions of engines, electric motors, or any mechanical device in operation are typically unwanted. Such vibrations could be caused by imbalances in the parts, uneven friction. Careful designs usually minimize unwanted vibrations, the studies of sound and vibration are closely related. Sound, or pressure waves, are generated by vibrating structures, hence, attempts to reduce noise are often related to issues of vibration. Free vibration occurs when a system is set in motion with an initial input. Examples of this type of vibration are pulling a child back on a swing and letting go, or hitting a tuning fork, the mechanical system vibrates at one or more of its natural frequencies and damps down to motionlessness. Forced vibration is when a disturbance is applied to a mechanical system. The disturbance can be a periodic and steady-state input, a transient input, the periodic input can be a harmonic or a non-harmonic disturbance. Damped vibration, When the energy of a system is gradually dissipated by friction and other resistances. The vibrations gradually reduce or change in frequency or intensity or cease, Vibration testing is accomplished by introducing a forcing function into a structure, usually with some type of shaker. Alternately, a DUT is attached to the table of a shaker, Vibration testing is performed to examine the response of a device under test to a defined vibration environment. The measured response may be life, resonant frequencies or squeak. Squeak and rattle testing is performed with a type of quiet shaker that produces very low sound levels while under operation. For relatively low frequency forcing, servohydraulic shakers are used, for higher frequencies, electrodynamic shakers are used. Generally, one or more input or control points located on the DUT-side of a fixture is kept at a specified acceleration, other response points experience maximum vibration level or minimum vibration level

3.
Liquid
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A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a constant volume independent of pressure. As such, it is one of the four states of matter. A liquid is made up of tiny vibrating particles of matter, such as atoms, water is, by far, the most common liquid on Earth. Like a gas, a liquid is able to flow and take the shape of a container, most liquids resist compression, although others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, a distinctive property of the liquid state is surface tension, leading to wetting phenomena. The density of a liquid is usually close to that of a solid, therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, although liquid water is abundant on Earth, this state of matter is actually the least common in the known universe, because liquids require a relatively narrow temperature/pressure range to exist. Most known matter in the universe is in form as interstellar clouds or in plasma form within stars. Liquid is one of the four states of matter, with the others being solid, gas. Unlike a solid, the molecules in a liquid have a greater freedom to move. The forces that bind the molecules together in a solid are only temporary in a liquid, a liquid, like a gas, displays the properties of a fluid. A liquid can flow, assume the shape of a container, if liquid is placed in a bag, it can be squeezed into any shape. These properties make a suitable for applications such as hydraulics. Liquid particles are bound firmly but not rigidly and they are able to move around one another freely, resulting in a limited degree of particle mobility. As the temperature increases, the vibrations of the molecules causes distances between the molecules to increase. When a liquid reaches its point, the cohesive forces that bind the molecules closely together break. If the temperature is decreased, the distances between the molecules become smaller, only two elements are liquid at standard conditions for temperature and pressure, mercury and bromine. Four more elements have melting points slightly above room temperature, francium, caesium, gallium and rubidium, metal alloys that are liquid at room temperature include NaK, a sodium-potassium metal alloy, galinstan, a fusible alloy liquid, and some amalgams

4.
Gas
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Gas is one of the four fundamental states of matter. A pure gas may be made up of atoms, elemental molecules made from one type of atom. A gas mixture would contain a variety of pure gases much like the air, what distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer, the interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image. One type of commonly known gas is steam, the gaseous state of matter is found between the liquid and plasma states, the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention, high-density atomic gases super cooled to incredibly low temperatures are classified by their statistical behavior as either a Bose gas or a Fermi gas. For a comprehensive listing of these states of matter see list of states of matter. The only chemical elements which are stable multi atom homonuclear molecules at temperature and pressure, are hydrogen, nitrogen and oxygen. These gases, when grouped together with the noble gases. Alternatively they are known as molecular gases to distinguish them from molecules that are also chemical compounds. The word gas is a neologism first used by the early 17th-century Flemish chemist J. B. van Helmont, according to Paracelsuss terminology, chaos meant something like ultra-rarefied water. An alternative story is that Van Helmonts word is corrupted from gahst and these four characteristics were repeatedly observed by scientists such as Robert Boyle, Jacques Charles, John Dalton, Joseph Gay-Lussac and Amedeo Avogadro for a variety of gases in various settings. Their detailed studies ultimately led to a relationship among these properties expressed by the ideal gas law. Gas particles are separated from one another, and consequently have weaker intermolecular bonds than liquids or solids. These intermolecular forces result from interactions between gas particles. Like-charged areas of different gas particles repel, while oppositely charged regions of different gas particles attract one another, transient, randomly induced charges exist across non-polar covalent bonds of molecules and electrostatic interactions caused by them are referred to as Van der Waals forces. The interaction of these forces varies within a substance which determines many of the physical properties unique to each gas. A comparison of boiling points for compounds formed by ionic and covalent bonds leads us to this conclusion, the drifting smoke particles in the image provides some insight into low pressure gas behavior

5.
Sense
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A sense is a physiological capacity of organisms that provides data for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology, the nervous system has a specific sensory system or organ, dedicated to each sense. Humans have a multitude of senses, sight, hearing, taste, smell, and touch are the five traditionally recognized senses. However, what constitutes a sense is a matter of debate, leading to difficulties in defining what exactly a distinct sense is. Other animals also have receptors to sense the world around them, humans have a comparatively weak sense of smell and a stronger sense of sight relative to many other mammals while some animals may lack one or more of the traditional five senses. Some animals may also intake and interpret sensory stimuli in different ways. Some species of animals are able to sense the world in a way humans cannot, with some species able to sense electrical and magnetic fields. There is no agreement as to the number of senses because of differing definitions of what constitutes a sense. The senses are frequently divided into exteroceptive and interoceptive, Exteroceptive senses are senses that perceive the bodys own position, motion, external senses include the traditional five, sight, hearing, touch, smell and taste, as well as thermoception and possibly an additional weak magnetoception. Proprioceptive senses include nociception, equilibrioception, proprioception, interoceptive senses are senses that perceive sensations in internal organs. Non-human animals may possess senses that are absent in humans, such as electroreception and detection of polarized light, in Buddhist philosophy, Ayatana or sense-base includes the mind as a sense organ, in addition to the traditional five. This addition to the commonly acknowledged senses may arise from the psychological orientation involved in Buddhist thought, the mind considered by itself is seen as the principal gateway to a different spectrum of phenomena that differ from the physical sense data. This way of viewing the human sense system indicates the importance of internal sources of sensation and perception that complements our experience of the external world, there are two types of photoreceptors, rods and cones. Rods are very sensitive to light, but do not distinguish colors, cones distinguish colors, but are less sensitive to dim light. There is some disagreement as to whether this one, two or three senses. Neuroanatomists generally regard it as two senses, given that different receptors are responsible for the perception of color and brightness, the inability to see is called blindness. Blindness may result from damage to the eyeball, especially to the retina, damage to the nerve that connects each eye to the brain. Temporary or permanent blindness can be caused by poisons or medications, people with blindsight are usually not aware that they are reacting to visual sources, and instead just unconsciously adapt their behaviour to the stimulus

6.
Auditory system
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The auditory system is the sensory system for the sense of hearing. It includes both the organs and the auditory parts of the sensory system. The outer ear funnels sound vibrations to the eardrum, increasing the pressure in the middle frequency range. The middle-ear ossicles further amplify the vibration pressure roughly 20 times, the base of the stapes couples vibrations into the cochlea via the oval window, which vibrates the perilymph liquid and causes the round window to bulb out as the oval window bulges in. Vestibular and tympanic ducts are filled with perilymph, and the cochlear duct between them is filled with endolymph, a fluid with a very different ion concentrations and voltage. Vestibular duct perilymph vibrations bend organ of Corti outer cells causing prestin to be released in cell tips and this causes the cells to be chemically elongated and shrunk, and hair bundles to shift which, in turn, electrically effects the basilar membrane’s movement. These motors amplify the perilymph vibrations that initially incited them over 40-fold, since both motors are chemically driven they are unaffected by the newly amplified vibrations due to recuperation time. There are 4x more OHC than IHC, the basilar membrane is a wall where the majority of the IHC and OHC sit. Basilar membrane width and stiffness corresponds to the frequencies best sensed by the IHC, at the cochlea base the Basilar is at its narrowest and most stiff, at the cochlea apex it is at its widest and least stiff. The tectorial membrane supports the remaining IHC and OHC, tectorial membrane helps facilitate cochlear amplification by stimulating OHC and IHC. Tectorials width and stiffness parallels Basilars and similarly aids in frequency differentiation, the superior olivary complex, in pons, is the first convergence of the left and right cochlear pulses. SOC has 14 described nuclei, their abbreviation are used here, MSO determines the angle the sound came from by measuring time differences in left and right info. LSO normalizes sound levels between the ears, it uses the sound intensities to help determine sound angle, LNTB are glycine-immune, used for fast signalling. VLPO have the function as DPO, but act in a different area. PVO, CPO, RPO, VMPO, ALPO and SPON are various signalling and inhibiting nuclei, the trapezoid body is where most of the cochlear nucleus fibers decussate, this cross aids in sound localization. The CN breaks into ventral and dorsal regions, bushy cells transmit timing info, their shape averages timing jitters. Stellate cells encode sound spectra by spatial neural firing rates based on auditory input strength, octopus cells have close to the best temporal precision while firing, they decode the auditory timing code. DCN also receives info from VCN, fusiform cells integrate information to determine spectral cues to locations

7.
Mechanical wave
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A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium. While waves can move long distances, the movement of the medium of transmission—the material—is limited. Therefore, oscillating material does not move far from its equilibrium position. This energy propagates in the direction as the wave. Any kind of wave has a certain energy, mechanical waves can be produced only in media which possess elasticity and inertia. A mechanical wave requires an energy input. Once this initial energy is added, the travels through the medium until all its energy is transferred. In contrast, electromagnetic waves require no medium, but can travel through one. One important property of waves is that their amplitudes are measured in an unusual way. When this gets comparable to unity, significant nonlinear effects such as harmonic generation may occur, for example, waves on the surface of a body of water break when this dimensionless amplitude exceeds 1, resulting in a foam on the surface and turbulent mixing. Some of the most common examples of mechanical waves are waves, sound waves. There are three types of waves, transverse waves, longitudinal waves, and surface waves. Transverse waves cause the medium to vibrate at an angle to the direction of the wave or energy being carried by the medium. Transverse waves have two parts—the crest and the trough, the crest is the highest point of the wave and the trough is the lowest. The distance between a crest and a trough is half of wavelength, the wavelength is the distance from crest to crest or from trough to trough. To see an example, move an end of a Slinky to the left-and-right of the Slinky, light also has properties of a transverse wave, although it is an electromagnetic wave. Longitudinal waves cause the medium to vibrate parallel to the direction of the wave and it consists of multiple compressions and rarefactions. The rarefaction is the farthest distance apart in the longitudinal wave, the speed of the longitudinal wave is increased in higher index of refraction, due to the closer proximity of the atoms in the medium that is being compressed

8.
Brain
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The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the organs for senses such as vision. The brain is the most complex organ in a vertebrates body, in a human, the cerebral cortex contains approximately 15–33 billion neurons, each connected by synapses to several thousand other neurons. Physiologically, the function of the brain is to exert centralized control over the other organs of the body, the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment, the operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. This article compares the properties of brains across the range of animal species. It deals with the human brain insofar as it shares the properties of other brains, the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because more can be said about them in a human context. The most important is brain disease and the effects of brain damage, the shape and size of the brain varies greatly between species, and identifying common features is often difficult. Nevertheless, there are a number of principles of architecture that apply across a wide range of species. Some aspects of structure are common to almost the entire range of animal species, others distinguish advanced brains from more primitive ones. The simplest way to gain information about brain anatomy is by visual inspection, Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, and then sliced apart for examination of the interior. Visually, the interior of the consists of areas of so-called grey matter, with a dark color, separated by areas of white matter. Further information can be gained by staining slices of tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations. It is also possible to examine the microstructure of brain tissue using a microscope, the brains of all species are composed primarily of two broad classes of cells, neurons and glial cells. Glial cells come in types, and perform a number of critical functions, including structural support, metabolic support, insulation. Neurons, however, are considered the most important cells in the brain. The property that makes neurons unique is their ability to send signals to target cells over long distances

9.
Touch
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The somatosensory system is a part of the sensory nervous system. The somatosensory system is a system of sensory neurons and pathways that responds to changes at the surface or inside the body. The axons, of sensory neurons connect with, or respond to, sensory receptors are found all over the body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the cardiovascular system. Somatic senses are sometimes referred to as somesthetic senses, with the understanding that somesthesis includes the sense of touch, proprioception, the mapping of the body surfaces in the brain is called a cortical homunculus and plays a fundamental role in the creation of body image. This brain-surface map is not immutable, however, dramatic shifts can occur in response to stroke or injury. The four mechanoreceptors in the skin respond to different stimuli for short or long periods. Merkel cell nerve endings are found in the epidermis and hair follicles, they react to low vibrations. Due to a receptive field they are used in areas like fingertips the most, they are not covered. Tactile corpuscles react to moderate vibration and light touch and they are located in the dermal papillae, due to their reactivity they are primarily located in fingertips and lips. They respond in quick action potentials, unlike Merkel and they are responsible for the ability to read Braille and feel gentle stimuli. Lamellar corpuscles determine gross touch and distinguish rough and soft substances and they react in quick action potentials, especially to vibrations around 250 Hz. They are the most sensitive to vibrations, and have large receptor fields, pacinian reacts only to sudden stimuli so pressures like clothes that are always compressing their shape are quickly ignored. Bulbous corpuscles react slowly and respond to sustained skin stretch and they are responsible for the feeling of object slippage and play a major role in the kinesthetic sense and control of finger position and movement. Merkel and bulbous cells are myelinated, the rest are not, all of these receptors are activated upon pressures that squish their shape causing an action potential. All afferent touch/vibration info ascends the spinal cord via the posterior column-medial lemniscus pathway via gracilis or cuneatus, cuneatus sends signals to the cochlear nucleus indirectly via spinal grey matter, this info is used in determining if a perceived sound is just villi noise/irritation. All fibers cross in the medulla, the postcentral gyrus includes the primary somatosensory cortex collectively referred to as S1. BA3 receives the densest projections from the thalamus, bA3a is involved with the sense of relative position of neighboring body parts and amount of effort being used during movement. BA3b is responsible for distributing somato info, it projects texture info to BA1, region S2 divides into Area S2 and parietal ventral area

10.
Pinna (anatomy)
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The auricle or auricula is the visible part of the ear that resides outside of the head. It is also called the pinna, a term that is used more in zootomy and these hillocks develop into the folds of the auricle and gradually shift upwards and backwards to their final position on the head. En route accessory auricles may be left behind, the first three hillocks are derived from the 1st branchial arch and form the tragus, crus of the helix, and helix, respectively. Cutaneous sensation to these areas is via the nerve, the attendant nerve of the 1st branchial arch. The final three hillocks are derived from the branchial arch and form the antihelix, antitragus, and lobule. These portions of the ear are supplied by the cervical plexus and this explains why vesicles are classically seen on the auricle in herpes infections of the facial nerve. The auricles functions are to sound and transform it into directional. The auricle collects sound and, like a funnel, amplifies the sound, the filtering effect of the human pinnae preferentially selects sounds in the frequency range of human speech. Amplification of sound by the pinna, tympanic membrane and middle ear causes an increase in level of about 10 to 15 dB in a range of 1.5 kHz to 7 kHz. This amplification is an important factor in inner ear trauma resulting from elevated sound levels, due to its anatomy, the pinna largely eliminates a small segment of the frequency spectrum, this band is called the pinna notch. The pinna works differently for low and high frequency sounds, for low frequencies, it behaves similarly to a reflector dish, directing sounds toward the ear canal. For high frequencies, however, its value is thought to be more sophisticated, while some of the sounds that enter the ear travel directly to the canal, others reflect off the contours of the pinna first, these enter the ear canal after a very slight delay. This delay causes phase cancellation, virtually eliminating the frequency component whose wave period is twice the delay period, in the affected frequency band – the pinna notch – the pinna creates a band-stop or notch filtering effect. This filter typically affects sounds around 10 kHz, though it can affect any frequencies from 6 –16 kHz and it also is directionally dependent, affecting sounds coming from above more than those coming from straight ahead. This aids in sound localization. In animals the function of the pinna is to collect sound and it collects sound by acting as a funnel, amplifying the sound and directing it to the auditory canal. While reflecting from the pinna, sound also goes through a filtering process, in various species, the pinna can also signal mood and radiate heat

11.
Ear canal
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The ear canal, is a tube running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about 2.5 centimetres in length and 0.7 centimetres in diameter, the human ear canal is divided into two parts. The elastic cartilage part forms the third of the canal, its anterior and lower wall are cartilaginous, whereas its superior. The cartilage is the continuation of the framework of pinna. The bony part forms the two thirds. The bony part is shorter in children and is only a ring in the newborn. Size and shape of the canal vary among individuals, the canal is approximately 2.5 centimetres long and 0.7 centimetres in diameter. It has a form and runs from behind and above downward and forward. On the cross-section, it is of oval shape and these are important factors to consider when fitting earplugs. Due to its exposure to the outside world, the ear canal is susceptible to diseases. It plays an important role in the ear canal, assisting in cleaning and lubrication, and also provides some protection from bacteria, fungi. Excess or impacted cerumen can press against the eardrum and/or occlude the external auditory canal, archived from the original on 2014-01-01

12.
Filter (signal processing)
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In signal processing, a filter is a device or process that removes some unwanted components or features from a signal. Filtering is a class of processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal. Most often, this means removing some frequencies and not others in order to suppress interfering signals, however, filters do not exclusively act in the frequency domain, especially in the field of image processing many other targets for filtering exist. Correlations can be removed for certain components and not for others without having to act in the frequency domain. There are many different bases of classifying filters and these overlap in many different ways, Filters may be, linear or non-linear time-invariant or time-variant, also known as shift invariance. If the filter operates in a spatial domain then the characterization is space invariance, causal or not-causal, A filter is non-causal if its present output depends on future input. Filters processing time-domain signals in real time must be causal, Analog or digital discrete-time or continuous-time passive or active type of continuous-time filter infinite impulse response or finite impulse response type of discrete-time or digital filter. Linear continuous-time circuit is perhaps the most common meaning for filter in the signal processing world and these circuits are generally designed to remove certain frequencies and allow others to pass. Circuits that perform this function are generally linear in their response, any nonlinearity would potentially result in the output signal containing frequency components not present in the input signal. The modern design methodology for linear continuous-time filters is called network synthesis, some important filter families designed in this way are, Chebyshev filter, has the best approximation to the ideal response of any filter for a specified order and ripple. Butterworth filter, has a flat frequency response. Bessel filter, has a flat phase delay. Elliptic filter, has the steepest cutoff of any filter for a specified order, the difference between these filter families is that they all use a different polynomial function to approximate to the ideal filter response. This results in each having a different transfer function, another older, less-used methodology is the image parameter method. Filters designed by this methodology are archaically called wave filters, some important filters designed by this method are, Constant k filter, the original and simplest form of wave filter. M-derived filter, a modification of the constant k with improved cutoff steepness, high-pass filter – high frequencies are passed, low frequencies are attenuated. Band-pass filter – only frequencies in a band are passed. Band-stop filter or band-reject filter – only frequencies in a band are attenuated

13.
Vertical sound localization
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Sound localization refers to a listeners ability to identify the location or origin of a detected sound in direction and distance. The sound localization technique appeared with the progress that has been made in acoustic simulation techniques. It allows us to localize the sources by modeling a sound field that contains one or several sources. Therefore, with technique, people can obtain the hearing sense from any place in the sound field. It may also refer to the methods in engineering to simulate the placement of an auditory cue in a virtual 3D space. The sound localization mechanisms of the auditory system have been extensively studied. As a new discipline, sound localization involves psychological acoustics, physiological acoustics, artificial intelligence. Animals with the ability to localize sound have an evolutionary advantage. Sound is the result of mechanical vibrations traveling through a medium such as air or water. Through the mechanisms of compression and rarefaction, sound waves travel through the air, bounce off the pinna and concha of the exterior ear, human ear has a very complex physical structure. As shown in fig.1, it can be divided into three parts, outer, middle and inner, the outer ear consists of the auricle, external auditory meatus and eardrum. For human adults, the auditory canal is about 2.7 cm long with a diameter around 0.7 cm. When the external auditory canal is closed, the lowest resonant frequency is around 3060 Hz, due to the resonance effect in the external auditory canal, the sound will gain a 10 dB amplification. The eardrum is shaped as a cone about 0.1 mm thick and 69 square mm, at normal voice levels, the eardrum displacement is around 0.1 nm. In general, the outer ear serves two functions, localizing the sound source and amplifying the acoustic signals, the middle ear is a chain that consists of three pieces of ossicles, malleus, anvil and stapes. The function of middle ear is to change acoustic impedance and protect inner ear, the main part of inner ear is cochlea. It is the receiver of human auditory systems that convert the signals to nerve stimulation. More detailed information can be found in Ear, in vertebrates, inter-aural time differences are known to be calculated in the superior olivary nucleus of the brainstem

14.
Waveform
–
A waveform is the shape and form of a signal such as a wave moving in a physical medium or an abstract representation. In many cases the medium in which the wave propagates does not permit a direct observation of the true form, in these cases, the term waveform refers to the shape of a graph of the varying quantity against time. An instrument called an oscilloscope can be used to represent a wave as a repeating image on a screen. To be more specific, a waveform is depicted by a graph that shows the changes in a signals amplitude over the duration of recording. The amplitude of the signal is measured on the y -axis, most programs show waveforms to give the user a visual aid of what has been recorded. If the waveform is of low or high height, the recording was most likely conducted under conditions with a low or high input volume, respectively. From this example, it follows that the represented by the waveform is affected by both the input signal and conditions under which it is recorded. A periodic waveforms include these while t is time, λ is wavelength, the amplitude of the waveform follows a trigonometric sine function with respect to time. Square wave = { a, mod λ < duty − a and this waveform is commonly used to represent digital information. A square wave of constant period contains odd harmonics that decrease at −6 dB/octave, triangle wave =2 a π arcsin ⁡ sin ⁡2 π t − ϕ λ. It contains odd harmonics that decrease at −12 dB/octave, sawtooth wave =2 a π arctan ⁡ tan ⁡2 π t − ϕ2 λ. This looks like the teeth of a saw, found often in time bases for display scanning. It is used as the point for subtractive synthesis, as a sawtooth wave of constant period contains odd. Other waveforms are often called composite waveforms and can often be described as a combination of a number of waves or other basis functions added together. The Fourier series describes the decomposition of periodic waveforms, such that any periodic waveform can be formed by the sum of a set of fundamental, finite-energy non-periodic waveforms can be analyzed into sinusoids by the Fourier transform. AC waveform Arbitrary waveform generator Spectrum analyzer Waveform monitor Waveform viewer Wave packet Yuchuan Wei, common Waveform Analysis, A New And Practical Generalization of Fourier Analysis. Springer US, Aug 31,2000 Waveform Definition Hao He, Jian Li, Waveform design for active sensing systems, a computational approach. Solomon W. Golomb, and Guang Gong, Signal design for good correlation, for wireless communication, cryptography, and radar

15.
Impedance matching
–
In the case of a complex source impedance ZS and load impedance ZL, maximum power transfer is obtained when Z S = Z L ∗ where the asterisk indicates the complex conjugate of the variable. Impedance is the opposition by a system to the flow of energy from a source, for constant signals, this impedance can also be constant. For varying signals, it changes with frequency. The energy involved can be electrical, mechanical, acoustic, magnetic, the concept of electrical impedance is perhaps the most commonly known. Electrical impedance, like electrical resistance, is measured in ohms, in general, impedance has a complex value, this means that loads generally have a resistance component which forms the real part of Z and a reactance component which forms the imaginary part of Z. In simple cases the reactance may be negligible or zero, the impedance can be considered a pure resistance, in the following summary we will consider the general case when resistance and reactance are both significant, and the special case in which the reactance is negligible. Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance, if the source impedance, load impedance and transmission line characteristic impedance are purely resistive, then reflection-less matching is the same as maximum power transfer matching. Complex conjugate matching is used when maximum power transfer is required and this differs from reflection-less matching only when the source or load have a reactive component. If the source has a component, but the load is purely resistive, then matching can be achieved by adding a reactance of the same magnitude. This simple matching network, consisting of an element, will usually only achieve a perfect match at a single frequency. For wide bandwidth applications, a complex network must be designed. For two impedances to be complex conjugates their resistances must be equal, and their reactances must be equal in magnitude, in low-frequency or DC systems the reactances are zero, or small enough to be ignored. In this case, maximum power occurs when the resistance of the load is equal to the resistance of the source. Impedance matching is not always necessary, for example, if a source with a low impedance is connected to a load with a high impedance the power that can pass through the connection is limited by the higher impedance. This maximum-voltage connection is a configuration called impedance bridging or voltage bridging. In such applications, delivering a voltage is often more important than maximum power transfer. In older audio systems, the source and load resistances were matched at 600 ohms, one reason for this was to maximize power transfer, as there were no amplifiers available that could restore lost signal. Most modern audio circuits, on the hand, use active amplification and filtering

16.
Neuronal encoding of sound
–
The neuronal encoding of sound is the representation of auditory sensation and perception in the nervous system. The complexities of contemporary neuroscience are continually redefined, thus what is known now of the auditory system has changed in the recent times and thus conceivably in the next two years or so, much of this will change. This article traces the route that sound waves first take from generation at a source to their integration and perception by the auditory cortex. Sound waves are what physicists call longitudinal waves, which consist of propagating regions of high pressure, waveform is a description of the general shape of the sound wave. Waveforms are sometimes described by the sum of sinusoids, via Fourier analysis, amplitude is the size of the pressure variations in a sound wave, and primarily determines the loudness with which the sound is perceived. In a sinusoidal function such as C sin ⁡, C represents the amplitude of the sound wave, the frequency of a sound is defined as the number of repetitions of its waveform per second, and is measured in hertz, frequency is inversely proportional to wavelength. The wavelength of a sound is the distance between any two consecutive matching points on the waveform, the audible frequency range for young humans is about 20 Hz to 20 kHz. Hearing of higher frequencies decreases with age, limiting to about 16 kHz for adults, given the simple physics of sound, the anatomy and physiology of hearing can be studied in greater detail. The Outer ear consists of the pinna or auricle, and the auditory meatus, the fundamental function of this part of the ear is to gather sound energy and deliver it to the eardrum. Resonances of the external ear selectively boost sound pressure with frequency in the range 2–5 kHz, the pinna as a result of its asymmetrical structure is able to provide further cues about the elevation from which the sound originated. The vertical asymmetry of the pinna selectively amplifies sounds of higher frequency from high elevation thereby providing spatial information by virtue of its mechanical design. The middle ear plays a role in the auditory process. In other words, it is the transfer function that allows for efficient transfer of collected sound energy between two different media. The three small bones that are responsible for this process are the malleus, the incus. The impedance matching is done through via lever ratios and the ratio of areas of the tympanic membrane, furthermore, the ossicles are arranged in such a manner as to resonate at 700–800 Hz while at the same time protecting the inner ear from excessive energy. A certain degree of control is present at the middle ear level primarily through two muscles present in this anatomical region, the tensor tympani and the stapedius. These two muscles can restrain the ossicles so as to reduce the amount of energy that is transmitted into the ear in loud surroundings. The cochlea of the ear, a marvel of physiological engineering

17.
Depolarization
–
In biology, depolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell. Depolarization is essential to the function of cells, communication between cells, and the overall physiology of an organism. Most cells in higher organisms maintain an environment that is negatively charged relative to the cells exterior. This difference in charge is called the membrane potential. In the process of depolarization, the internal charge of the cell temporarily becomes more positive. This shift from a negative to a more positive membrane potential occurs during several processes, during an action potential, the depolarization is so large that the potential difference across the cell membrane briefly reverses polarity, with the inside of the cell becoming positively charged. The change in charge typically occurs due to an influx of ions into a cell. The opposite of a depolarization is called a hyperpolarization, usage of the term depolarization in biology differs from its use in physics. In physics it refers instead to situations in which any form of polarity changes to a value of zero, the process of depolarization is entirely dependent upon the intrinsic electrical nature of most cells. When a cell is at rest, the cell maintains what is known as a resting potential, the resting potential generated by nearly all cells results in the interior of the cell having a negative charge compared to the exterior of the cell. To maintain this electrical imbalance, microscopic positively and negatively charged particles called ions are transported across the plasma membrane. The resting potential must be established within a cell before the cell can be depolarized, there are many mechanisms by which a cell can establish a resting potential, however there is a typical pattern of generating this resting potential that many cells follow. The cell uses ion channels, ion pumps, and voltage gated ion channels to generate a negative resting potential within the cell, however, the process of generating the resting potential within the cell also creates an environment outside of the cell that favors depolarization. The sodium potassium pump is largely responsible for the optimization of conditions on both the interior and the exterior of the cell for depolarization, additionally, despite the high concentration of positively-charged potassium ions, most cells contain internal components, which accumulate to establish a negative inner-charge. After a cell has established a potential, that cell has the capacity to undergo depolarization. During depolarization, the charge within the cell rapidly shifts from negative to positive, for this rapid change to take place within the interior of the cell, several events must occur along the plasma membrane of the cell as well. While the sodium potassium pump continues to work, the voltage gated ion channels that had closed while the cell was at resting potential have been opened by an electrical stimulus. As the sodium rushes back into the cell the positive sodium ions raise the charge inside the cell from negative to positive, once the interior of the cell becomes positively charged, depolarization of the cell is complete

18.
Hair cells
–
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates. Through mechanotransduction, hair cells detect movement in their environment, in mammals, the auditory hair cells are located within the spiral organ of Corti on the thin basilar membrane in the cochlea of the inner ear. They derive their name from the tufts of stereocilia called hair bundles that protrude from the surface of the cell into the fluid-filled cochlear duct. Mammalian cochlear hair cells are of two anatomically and functionally distinct types, known as outer, and inner hair cells, damage to these hair cells results in decreased hearing sensitivity, and because the inner ear hair cells cannot regenerate, this damage is permanent. However, other organisms, such as the frequently studied zebrafish, the human cochlea contains on the order of 3,500 inner hair cells and 12,000 outer hair cells at birth. Outer hair cells mechanically amplify low-level sound that enters the cochlea, the amplification may be powered by the movement of their hair bundles, or by an electrically driven motility of their cell bodies. This so-called somatic electromotility amplifies sound in all land vertebrates and it is affected by the closing mechanism of the mechanical sensory ion channels at the tips of the hair bundles. The deflection of the hair-cell stereocilia opens mechanically gated ion channels that allow any small, unlike many other electrically active cells, the hair cell itself does not fire an action potential. Instead, the influx of ions from the endolymph in the scala media depolarizes the cell. This receptor potential opens voltage gated calcium channels, calcium ions enter the cell. The neurotransmitters diffuse across the space between the hair cell and a nerve terminal, where they then bind to receptors and thus trigger action potentials in the nerve. In this way, the sound signal is converted into an electrical nerve signal. Repolarization of hair cells is done in a special manner, the perilymph in the scala tympani has a very low concentration of positive ions. The electrochemical gradient makes the positive ions flow through channels to the perilymph and this leakage causes a tonic release of neurotransmitter to the synapses. It is thought that this release is what allows the hair cells to respond so quickly in response to mechanical stimuli. The quickness of the cell response may also be due to the fact that it can increase the amount of neurotransmitter release in response to a change as little as 100 μV in membrane potential. In mammalian outer hair cells, the receptor potential triggers active vibrations of the cell body, outer hair cells are found only in mammals. While hearing sensitivity of mammals is similar to that of classes of vertebrates, without functioning outer hair cells

19.
Action potential
–
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of cells, called excitable cells, which include neurons, muscle cells. In other types of cells, their function is to activate intracellular processes. In muscle cells, for example, a potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas, they provoke release of insulin, action potentials in neurons are also known as nerve impulses or spikes, and the temporal sequence of action potentials generated by a neuron is called its spike train. A neuron that emits an action potential is often said to fire, action potentials are generated by special types of voltage-gated ion channels embedded in a cells plasma membrane. When the channels open they allow a flow of sodium ions, which changes the electrochemical gradient. This then causes more channels to open, producing an electric current across the cell membrane. The process proceeds explosively until all of the ion channels are open. The rapid influx of ions causes the polarity of the plasma membrane to reverse. As the sodium channels close, sodium ions can no longer enter the neuron, potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, in animal cells, there are two primary types of action potentials. One type is generated by voltage-gated sodium channels, the other by voltage-gated calcium channels, sodium-based action potentials usually last for under one millisecond, whereas calcium-based action potentials may last for 100 milliseconds or longer. In some types of neurons, slow calcium spikes provide the force for a long burst of rapidly emitted sodium spikes. In cardiac muscle cells, on the hand, an initial fast sodium spike provides a primer to provoke the rapid onset of a calcium spike. Nearly all cell membranes in animals, plants and fungi maintain a voltage difference between the exterior and interior of the cell, called the membrane potential. A typical voltage across a cell membrane is –70 mV. In most types of cells the membrane potential usually stays fairly constant, some types of cells, however, are electrically active in the sense that their voltages fluctuate over time

20.
Cochlear nucleus
–
The cochlear nuclear complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus and the dorsal cochlear nucleus. The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus is layered, at the nerve root the fibers branch to innervate the ventral cochlear nucleus and the deep layer of the dorsal cochlear nucleus. All acoustic information thus enters the brain through the cochlear nuclei, the outputs from the cochlear nuclei are received in higher regions of the auditory brainstem. The cochlear nuclei are located at the side of the brainstem, spanning the junction of the pons. The ventral cochlear nucleus on the aspect of the brain stem. The dorsal cochlear nucleus, also known as the tuberculum acusticum or acoustic tubercle, curves over the VCN, the VCN is further divided by the nerve root into the posteroventral cochlear nucleus and the anteroventral cochlear nucleus. The major input to the nucleus is from the auditory nerve. The auditory nerve fibers form an organized system of connections according to their peripheral innervation of the cochlea. Axons from the ganglion cells of the lower frequencies innervate the ventrolateral portions of the ventral cochlear nucleus. The axons from the higher frequency organ of corti hair cells project to the portion of the ventral cochlear nucleus. The mid frequency projections end up in between the two extremes, in this way the organization that is established in the cochlea is preserved in the cochlear nuclei. In contrast with the VCN that receives all acoustic input from the auditory nerve, the DCN is therefore in a sense a second order sensory nucleus. The cochlear nuclei have long thought to receive input only from the ipsilateral ear. There is evidence, however, for stimulation from the ear via the contralateral CN. There are three major fiber bundles, axons of cochlear nuclear neurons, that information from the cochlear nuclei to targets that are mainly on the opposite side of the brain. Through the medulla, one goes to the contralateral superior olivary complex via the trapezoid body. This pathway is called the ventral acoustic stria, another pathway, called the dorsal acoustic stria, rises above the medulla into the pons where it hits the nuclei of the lateral lemniscus along with its kin, the intermediate acoustic stria. The IAS decussates across the medulla, before joining the ascending fibers in the lateral lemniscus

21.
Tectum
–
The tectum is a region of the brain, specifically the dorsal part of the midbrain. This is contrasted with the tegmentum, which refers to the ventral to the ventricular system. The tectum is responsible for auditory and visual reflexes and it is derived in embryonic development from the alar plate of the neural tube. In adult humans, it consists of the inferior and the superior colliculi. The superior colliculus is involved in visual processing and control of eye movements. In non-mammalian vertebrates it serves as the visual area of the brain. The inferior colliculus is involved in auditory processing and it receives input from various brain stem nuclei and projects to the medial geniculate nucleus of the thalamus, which relays auditory information to the primary auditory cortex. Collectively the colliculi are referred to as the corpora quadrigemina, the term tectal plate is used to describe the junction of the gray and white matter in the embryo. List of regions in the human brain Tectospinal tract Diagram Photo Anatomy diagram,13048. 000-3, archived from the original on 2014-01-01

22.
Medial geniculate nucleus
–
The medial geniculate nucleus or medial geniculate body is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus and the auditory cortex. It is made up of a number of sub-nuclei that are distinguished by their morphology and density, by their afferent and efferent connections. It is thought that the MGB influences the direction and maintenance of attention, the MGB has three major divisions, ventral, dorsal and medial. Whilst the VMGB is specific to information processing, the DMGB and MMGB also receive information from non-auditory pathways. The long axis of the cells lie parallel to each other running superior-inferiorly with the dendritic trees of cells within the same iso-frequency band overlapping. This is similar to the organization of the ICC. The dendrites of relay cells form a synaptic nest with ascending axons from the inferior colliculus, in this synapse, relay cells are excited by input from the IC axons. At the same time, they are inhibited by dendritodendritic synapses from the interneurons of the VMGB and this type of synaptic nesting is characteristic of other regions in the thalamus as well. Intrathalamic Interneurons, These interneurons provide inhibitory input to the cells at the synaptic nests. The target of their axons however, is not clear, some interneurons appear to target relay cells, while others target other interneurons. There is also at least one type of interneuron that appears to not be involved in the synaptic nests, the VMGB is thought to be primarily responsible for relaying frequency, intensity and binaural information to the cortex. The responses in the VMGB appear to be organized in a similar way to those in the IC. The primary difference being that the bands are arranged such that lateral regions are most responsive to low frequencies. Spatiotopic and modulotopic maps however have not been supported by mammalian studies. Both monaural and binaural cells exist in the MGB, the monaural cells are primarily responsive to sound in the contralateral hemifield. Binaural cells are similar to the EE or EI type found in the IC. Several sub-nuclei have been identified based on morphology, no frequency-specific layering has been found in the DMGB. Many types of responses are present in the DMGB that appear to vary by sub-nuclei, generally, the responses are broadly tuned, but some cells appear to respond only to complex stimuli

Sound
–
In physics, sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a transmission medium such as air or water. In physiology and psychology, sound is the reception of such waves, humans can hear sound waves with frequencies between about 20 Hz and 20 kHz. Sound above 20 kHz is ultrasound an

1.
A drum produces sound via a vibrating membrane.

2.
Audio engineers in R&D design audio equipment

3.
U.S. Navy F/A-18 approaching the sound barrier. The white halo is formed by condensed water droplets thought to result from a drop in air pressure around the aircraft (see Prandtl-Glauert Singularity).

4.
Human ear

Vibration
–
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin vibrationem, the oscillations may be periodic, such as the motion of a pendulum—or random, such as the movement of a tire on a gravel road. Vibration can be desirable, for example, the motion of a fork, the reed in a woodwind instru

1.
Car Suspension: designing vibration control is undertaken as part of acoustic, automotive or mechanical engineering.

2.
One of the possible modes of vibration of a circular drum (see other modes).

Liquid
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A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a constant volume independent of pressure. As such, it is one of the four states of matter. A liquid is made up of tiny vibrating particles of matter, such as atoms, water is, by far, the most common liquid on Earth. Like a gas, a liquid is able to flo

1.
The formation of a spherical droplet of liquid water minimizes the surface area, which is the natural result of surface tension in liquids.

2.
Thermal image of a sink full of hot water with cold water being added, showing how the hot and the cold water flow into each other.

3.
Surface waves in water

Gas
–
Gas is one of the four fundamental states of matter. A pure gas may be made up of atoms, elemental molecules made from one type of atom. A gas mixture would contain a variety of pure gases much like the air, what distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a color

1.
Drifting smoke particles provide clues to the movement of the surrounding gas.

2.
Gas phase particles (atoms, molecules, or ions) move around freely in the absence of an applied electric field.

Sense
–
A sense is a physiological capacity of organisms that provides data for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology, the nervous system has a specific sensory system or organ, dedicated to each sense. Humans have a multi

1.
Five senses and the respective sensory organs

2.
An allegory of five senses. Still Life by Pieter Claesz, 1623. The painting illustrates the senses through musical instruments, a compass, a book, food and drink, a mirror, incense and an open perfume bottle. The tortoise may be an illustration of touch or an allusion to the opposite,(the tortoise isolating in its shell)

3.
Detail of The Senses of Hearing, Touch and Taste, Jan Brueghel the Elder, 1618

4.
In this painting by Pietro Paolini, each individual represents one of the five senses.

Auditory system
–
The auditory system is the sensory system for the sense of hearing. It includes both the organs and the auditory parts of the sensory system. The outer ear funnels sound vibrations to the eardrum, increasing the pressure in the middle frequency range. The middle-ear ossicles further amplify the vibration pressure roughly 20 times, the base of the s

1.
Auditory ossicles from a deep dissection of the tympanic cavity

2.
Anatomy of the human ear. (The length of the auditory canal is exaggerated in this image)

Mechanical wave
–
A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium. While waves can move long distances, the movement of the medium of transmission—the material—is limited. Therefore, oscillating material does not move far from its equilibrium position. This energy propagates in the direction as the wave.

1.
Ripple in water is a surface wave.

Brain
–
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head, usually close to the organs for senses such as vision. The brain is the most complex organ in a vertebrates body, in a human, the cerebral cortex contains approximately 15–33 billion neurons, each

1.
A chimpanzee brain

2.
Cross section of the olfactory bulb of a rat, stained in two different ways at the same time: one stain shows neuron cell bodies, the other shows receptors for the neurotransmitter GABA.

3.
Neurons generate electrical signals that travel along their axons. When a pulse of electricity reaches a junction called a synapse, it causes a neurotransmitter chemical to be released, which binds to receptors on other cells and thereby alters their electrical activity.

4.
Neurons often have extensive networks of dendrites, which receive synaptic connections. Shown is a pyramidal neuron from the hippocampus, stained for green fluorescent protein.

Touch
–
The somatosensory system is a part of the sensory nervous system. The somatosensory system is a system of sensory neurons and pathways that responds to changes at the surface or inside the body. The axons, of sensory neurons connect with, or respond to, sensory receptors are found all over the body including the skin, epithelial tissues, muscles, b

2.
Touch can result in many different physiological reactions. Here, a baby laughs at being tickled by an older sister.

Pinna (anatomy)
–
The auricle or auricula is the visible part of the ear that resides outside of the head. It is also called the pinna, a term that is used more in zootomy and these hillocks develop into the folds of the auricle and gradually shift upwards and backwards to their final position on the head. En route accessory auricles may be left behind, the first th

1.
The fennec fox uses its distinctive oversized pinnae to radiate excess heat and to amplify the sound of small prey burrowing under the desert sand

2.
The auricula. Lateral surface.

3.
To an impala, the pinna is useful in collecting sound

4.
Left human pinna

Ear canal
–
The ear canal, is a tube running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about 2.5 centimetres in length and 0.7 centimetres in diameter, the human ear canal is divided into two parts. The elastic cartilage part forms the third of the canal, its anterior and lower wall are cartila

1.
Lateral head anatomy detail.Facial nerve dissection.

2.
Anatomy of the human ear.

Filter (signal processing)
–
In signal processing, a filter is a device or process that removes some unwanted components or features from a signal. Filtering is a class of processing, the defining feature of filters being the complete or partial suppression of some aspect of the signal. Most often, this means removing some frequencies and not others in order to suppress interf

1.
linear or non-linear

Vertical sound localization
–
Sound localization refers to a listeners ability to identify the location or origin of a detected sound in direction and distance. The sound localization technique appeared with the progress that has been made in acoustic simulation techniques. It allows us to localize the sources by modeling a sound field that contains one or several sources. Ther

2.
Interaural Level Difference (ILD) between left ear (left) and right ear (right). [sound source: a sweep from right]

Waveform
–
A waveform is the shape and form of a signal such as a wave moving in a physical medium or an abstract representation. In many cases the medium in which the wave propagates does not permit a direct observation of the true form, in these cases, the term waveform refers to the shape of a graph of the varying quantity against time. An instrument calle

1.
Sine, square, triangle, and sawtooth waveforms

Impedance matching
–
In the case of a complex source impedance ZS and load impedance ZL, maximum power transfer is obtained when Z S = Z L ∗ where the asterisk indicates the complex conjugate of the variable. Impedance is the opposition by a system to the flow of energy from a source, for constant signals, this impedance can also be constant. For varying signals, it ch

1.
Source and load circuit impedance

Neuronal encoding of sound
–
The neuronal encoding of sound is the representation of auditory sensation and perception in the nervous system. The complexities of contemporary neuroscience are continually redefined, thus what is known now of the auditory system has changed in the recent times and thus conceivably in the next two years or so, much of this will change. This artic

1.
Graph of a simple sine wave

2.
Flowchart of sound passage - outer ear

3.
Flowchart of sound passage - middle ear

4.
Flowchart of sound passage - inner ear

Depolarization
–
In biology, depolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell. Depolarization is essential to the function of cells, communication between cells, and the overall physiology of an organism. Most cells in higher organisms maintain an env

1.
Membrane potential through depolarization and repolarization

2.
Voltage gated ion channel opening during depolarization

3.
Structure of a neuron

4.
Summation of stimuli at an axon hillock

Hair cells
–
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates. Through mechanotransduction, hair cells detect movement in their environment, in mammals, the auditory hair cells are located within the spiral organ of Corti on the thin basilar membrane in the cochlea of the inner ear. They d

Action potential
–
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of cells, called excitable cells, which include neurons, muscle cells. In other types of cells, their function is to activate intracellu

1.
In saltatory conduction, an action potential at one node of Ranvier causes inwards currents that depolarize the membrane at the next node, provoking a new action potential there; the action potential appears to "hop" from node to node.

3.
Figure.1: Cable theory's simplified view of a neuronal fiber. The connected RC circuits correspond to adjacent segments of a passive neurite. The extracellular resistances r e (the counterparts of the intracellular resistances r i) are not shown, since they are usually negligibly small; the extracellular medium may be assumed to have the same voltage everywhere.

4.
The giant axons of the European squid (Loligo vulgaris) were crucial for scientists to understand the action potential.

Cochlear nucleus
–
The cochlear nuclear complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus and the dorsal cochlear nucleus. The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus is layered, at the nerve root the fibers branch to innervate the ventral cochlear nucleus and the deep layer of the dorsal

1.
Dissection of brainstem. Dorsal view. ("Cochlear nucleus" is labeled on left, fifth from the bottom.)

Tectum
–
The tectum is a region of the brain, specifically the dorsal part of the midbrain. This is contrasted with the tegmentum, which refers to the ventral to the ventricular system. The tectum is responsible for auditory and visual reflexes and it is derived in embryonic development from the alar plate of the neural tube. In adult humans, it consists of

1.
Midbrain tectum

2.
Deep dissection of brain-stem. Lateral view.

Medial geniculate nucleus
–
The medial geniculate nucleus or medial geniculate body is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus and the auditory cortex. It is made up of a number of sub-nuclei that are distinguished by their morphology and density, by their afferent and efferent connections. It is thought that the MGB inf

2.
Disposable foam earplugs: with coins for scale (top) and inserted into the wearer's ear.

3.
Musicians' earplugs. The grey end caps contain an acoustic transmission line with a damper (attenuator) at the end while the domed flanges form a seal in outer part of the ear canal. The output port can just be seen as a small hole at the near end of the left plug

4.
Earplugs made from silicone rubber. The hole seen in the left plug is the input port and extends as far as the central flange where the attenuation occurs

3.
Illustration of some of the first bulky vacuum tube hearing aids from 1933

4.
A sign in a train station explains that the public announcement system uses a "Hearing Induction Loop" (Audio induction loop). Hearing aid users can use a telecoil (T) switch to hear announcements directly through their hearing aid receiver.

1.
A resonant-reed frequency meter, an obsolete device used from about 1900 to the 1940s for measuring the frequency of alternating current. It consists of a strip of metal with reeds of graduated lengths, vibrated by an electromagnet. When the unknown frequency is applied to the electromagnet, the reed which is resonant at that frequency will vibrate with large amplitude, visible next to the scale.

2.
As time elapses – represented here as a movement from left to right, i.e. horizontally – the five sinusoidal waves shown vary regularly (i.e. cycle), but at different rates. The red wave (top) has the lowest frequency (i.e. varies at the slowest rate) while the purple wave (bottom) has the highest frequency (varies at the fastest rate).

2.
The bats are (from top to bottom and left to right): the greater mouse-eared bat, the lesser horseshoe bat, the brown long-eared bat, the common pipistrelle, the greater noctule bat, and the barbastelle, Romanian post miniature sheet, 2003

1.
Spectrogram of Pipistrellus Bat vocalizations. Detail is shown as the pulses transition to a faster repetition rate. The bat appears to use a hybrid pulse which combines a sharp falling frequency chirp with an extended constant frequency tail. Such a waveform may offer combined benefits of range estimation as well as Doppler shift detection. [citation needed]

2.
A depiction of the ultrasound signals emitted by a bat, and the echo from a nearby object.